Free hanging solid panels can absorb sound

Posted on 4 July 20244 July 2024 by ateliercrescendo

Solid free hanging panels can absorb sound

Did you know that panels, made of a solid and non-porous materials, located freely in the air can absorb sound?

You may not need to know this for all projects requiring acoustic design, but it could be useful if your project involves suspended, free hanging and/or free standing elements like canopies or shells around music stages.

And because it may sound counter intuitive, this short article is here to explain:

Why solid panels can absorb sound;
How to specify them;

Enjoy the read!

(See at the end the references used to confirm the concepts)

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Are you looking to find examples of acoustic solutions and products available on the market?

Visit the Acoustic Design Catalogue by clicking on the button below.

access the catalogue here

Why can solid free hanging panels absorb sound ?

A solid, non-porous panel hanging freely can absorb sound.

However, absorption is minimal at mid and high frequencies (above 500 Hz). It becomes more significant at low frequencies (below 500 Hz).

This is due to ‘mass reactance’.

When sound waves hit a panel, they cause it to move and vibrate, which creates a loss of acoustic energy.

Consequently, panels absorb more energy at low frequencies.

Practically, this happens because larger air particle movements are needed to move a panel.

Note:

You may need to read Fundamental concepts of sounds to understand the relationship between frequencies and the motion of air particules.

How to specify solid free hanging panels?

Obviously, a specification needs to consider all factors that influence a panel’s sound absorption.

One formula estimates the resonant frequency of a simply supported panel:

Note:

Why is it useful to calculate the resonant frequency of a panel?

Because it is the frequency at which a panel moves the most and absorbs the most sound.

with:

B the bending stiffness of the panel/material;
m the mass of the panel;
a and b respectively the width and the length of the panel;
i and n correspond to the harmonics of resonance, that you don’t need to understand in depth for this article.

This formula may be daunting for you, but you can note the influence of different properties.

Influence of the

mass ‘m’

The higher the mass the lower the resonant frequency.

(And vice versa, the lower the mass the higher the resonant frequency.)

This means that a good specification should reference:

the material used for the panel(s);
the density of the material(s);
the thickness of the panel(s);
and any other factors influencing the mass of a panels.

Influence of the

stiffness ‘B’

The higher the (bending) stiffness, the higher the resonant frequency.

(And vice versa, the lower the stiffness, the higher the resonant frequency.)

This means that a good specification should reference:

(again) the material used for the panel(s) ;
the mounting and fixing conditions of the panel(s);
and any other factors influencing the mass of the panel(s) like how curved they are (curving a material generally makes it stiffer).

Influence of the panel

dimensions

‘a’ and ‘b’

The larger the panel, the lower the resonant frequency.

(And vice versa, the smaller the panel, the higher the resonant frequency.)

This means that a good specification should reference the dimensions of the panel(s).

References

See below the references used to confirm the above concepts:

Architectural Acoustics – Second Edition – Marshall Long – Academic Press
Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition – Trevor Cox, Peter D’Antonio
Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen

Porous sound absorbers explained

Posted on 1 May 202423 May 2024 by ateliercrescendo

Porous sound absorbers

explained

Most (99% actually) materials used in construction to absorb sound are porous.

And do you know:

the 3 differents types of porous absorbers?
the importance of their porosity and air flow resistivity?
the influence of their thickness?
the influence of their density?
the influence of the mounting conditions?
the impact of the materials used to cover them?

In this post, we answer all the above questions.

Enjoy the read!

(See at the end the references used to confirm the concepts)

You want to be notified when new posts are published? Why don’t you subscribe to Atelier Crescendo’s newsletter by clicking here?

Are you looking to find examples of acoustic solutions and products available on the market?

Visit the Acoustic Design Catalogue by clicking on the button below.

access the catalogue here

What are porous sound absorbers ?

Primarily, Porous absorbers are used to diminish sound reverberation in rooms or to mitigate noise transmission when installed in cavities.

The sound absorption is achieved when sound passes through networks of interconnected pores, inducing viscous and thermal effects that dissipate acoustic energy as heat.

There are three types of porous absorbers.

Fibrous

absorbers

The sound absorption properties of fibrous absorbers stem from the arrangement of overlapping fibers, creating interconnected air pockets (the pores).

Examples of fibrous absorbers include:

mineral wool

wood fibre

hemp

textile fibres

felt

wool

Cellular

absorbers

The sound absorption properties of cellular absorbers arise from pores either isolated from their neighbours (known as “closed” pores or cells) or interconnected with their neighbours (known as “open” pores or cells).

Examples of cellular absorbers include:

PU foam

PET foam

GRANULar

absorbers

The sound absorption properties of granular absorbers originate from the air pockets (the pores) formed between tiny grains densely packed together.

Examples of granular absorbers include:

concrete

gravel

soils

Note

The shape and dimensions of the pores and fibres also significantly influence the sound absorption

Porosity and air flow resistivity of porous sound absorbers ?

The sound absorption of a material is closely tied to its porosity and flow resistivity, representing the resistance of the air as it traverses the material.

Hence, higher porosity correlates with greater sound absorption.

Porosity is defined as the ratio of pore volume to the matrice/medium volume.

Porosity of some construction materials

Material

Typical porosities (%)

Felt

0,83 – 0,97

Hemp

0,99

Mineral wool

0,92 – 0,99

Open cell acoustic foams

(e.g. PU)

0,93 – 0,995

Rubber crumb

0,44 – 0,54

Sand

0,39 – 0,44

Marble

≈ 0,005

Influence of the thickness on the sound absorption?

When located on solid/hard surfaces, the sound absorption tends to increase with the thickness of the porous material, particularly at low and mid frequencies.

See below the sound absorption of fibreglass (53 kg/m³ density) samples of different thicknesses, fixed to a hard surface.

Sound absorption of fibreglass (53 kg/m³ density) samples of 25 mm, 50 mm, 75 mm and 100 mm thicknesses (fixed to a hard surface)

Why is that

With increasing thickness, more material exists away from the surface, where the particules’ velocity (i.e. energy) is higher than at the wall.

This leads to enhanced absorption, particularly for lower frequencies (or larger wavelengths).

If you need a refresher about frequencies and wavelengths, read this post.

Influence of the density on the sound absorption?

Similarly to the thickness, the sound absorption at low and mid frequencies typically increases with the density of a fibrous absorber.

Below are the sound absorption coefficients of mineral wool blanket samples with varying densities.

Sound absorption of mineral wool of 16 kg/m³ , 24 kg/m³, 48 kg/m³and 60 kg/m³densities (50 mm fixed to a hard surface)

Influence of the mounting conditions on the sound absorption?

The sound absorption of a material greatly depends on how it is mounted.

Remember the post about Sound absorption basics? We introduced ISO EN 354 (2003 – Acoustics Measurement of sound absorption in a reverberation room mention) setting measurement standards for various mounting conditions.

For instance, consider acoustic ceiling tiles, which can be either directly fixed to a soffit or suspended.

While the sound absorption is comparable at mid and high frequencies, it is notably higher at low frequencies when the ceiling tiles are suspended (see the graph below).

Sound absorption of acoustic tiles directly applied to a hard surface or suspended

Also, in Suspended acoutic rafts explained, we showed that the following aspect influence the sound absorption characteristics a raft systems:

the sound absorption of the material installed in a room
the size of the rafts
the spacing between the rafts
the distance between the rafts and the hard surface above.

Influence of the cover on the sound absorption?

In many cases, sound absorbers are covered or protected by materials such as:

fabric
felt
plastic
paint
perforated sheet material
slats

Some of these materials possess specific air flow resistivity and may prevent sound from passing through. Thereby reducing the sound absorption especially at the higher frequencies. Less so at low frequencies.

This is why some materials, like paint, should be avoided as they block the pores of the materials (unless it is a non-bridging paint). See below different performances for different paint applications.

Influence of paint treatment on the absorption of a sound absorber

The same applies to glue behind cloths.

Cloth, perforated sheet materials and slats are typically preferred for protecting the sound absorbers against impact and/or small particles.

References

See below the references used to confirm the above concepts:

Noise Control in Building Services – Sound Research Laboratories Ltd – Pergamon Press
Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers
Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition – Trevor Cox, Peter D’Antonio
Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen

Sound absorption classes explained

Posted on 9 April 202420 May 2024 by ateliercrescendo

Class A Class B Class C Class D Class E - practical sound absorption coefficient - sound absorption class calculation - sound absorption - acoustics - acoustic consultant

Sound absorption classes

explained

Sound absorption classes are often used to specify sound absorption materials to be installed in buildings.

But have you ever wondered what they are exactly, what they represent and how they are established/calculated?

If you want to know, this short article has been created for you, with some graphics to make you understand the calculation process.

Enjoy the read!

You want to be notified when new posts are published? Why don’t you subscribe to Atelier Crescendo’s newsletter by clicking here?

Are you looking to find examples of acoustic solutions and products available on the market?

Visit the Acoustic Design Catalogue by clicking on the button below.

access the catalogue here

What are sound absorption classes ?

The classification is a method, among others, to categorise the sound absorption performance of absorbers, particularly for construction materials.

Note

You can find the details of this classification in ISO 11654 (2023) – Sound absorbers for use in buildings – Rating of sound absorption.

It is simpler and easier for non-acousticians to grasp since it doesn’t involve numbers.

There are five classes ranging from Class E (lowest performance) to Class A (highest performance).

How are sound absorption classes established ?

Establishing the sound absorption class of a material involves two main steps:

Converting the frequency-dependent sound absorption performance to a single-number value.
Assigning the single-number performance to the corresponding class.

From frequency-dependent performance to single number value

Sound absorption data is typically measured in third-octave bands.

(if you need to understand about frequencies read this article, and frequency bands read this article)

The initial conversion involves transitioning from the sound absorption coefficient measured in third-octave bands to the Practical Sound Absorption Coefficient (α_p) in octave bands (as introduced in this article).

From there, we calculate the Weighted Sound Absorption Coefficient (α_w), which is a single number determined as detailed below.

We begin with two curves:

the Practical Sound Absorption Coefficient (α_p) values calculated (in yellow below).
A curve known as the reference curve, with specific values for each octave band between 250 Hz and 4000 Hz (in black below).

The second step involves calculating the “unfavourable deviations” for each octave band.

Note 1:

An unfavorable deviation occurs when, for a given octave, the measured value is

less

than the value of the reference curve.

And vice versa, a ‘favourable’ deviation occurs when, for a given octave, the measured value is

than the value of the reference curve.

Note 2:

In the calculations, we only consider

unfavourable deviations.

Then, we incrementally adjust the reference curve in 0.05 steps toward the measured value until the sum of unfavorable deviations is ≤ 0.10.

Once this criterion (≤ 0.10) is met, the performance (α_w) for the material is determined by the value of the shifted reference curve at 500 Hz.

Another important point to note is if a measured value exceeds the reference value by more than 0.25 (even after the shifting process), a shape indicator is added to the weighted sound absorption performance as follows:

L, like α_w (L), if excess occurs at 250 Hz
M, like α_w (M), if excess occurs at 500 and 1000 Hz
H, like α_w (H), if excess occurs at 2000 and 4000 Hz

From single number performance to sound absorption class

Once the single number is calculated, we associate it with a class using the table below.

The figure below presents the reference curves corresponding to each class (you have probably seen this type of graph before).

Sound absorption basics

Posted on 26 March 202420 May 2024 by ateliercrescendo

Sound absorption basics

Often, we, acousticians, are asked about the fundamentals of sound absorption.

We are also asked to explain sound absorption data received by suppliers & manufacturers of sound absorbing materials.

If you you need clarification on sound absorption, we wrote this article for you.

It explains

what a sound absorber is
how we rate the sound absorption capacity of materials.
how we measure the sound absorption coefficient of materials.

Enjoy the read!

(See at the end the references used to confirm the concepts)

Are you looking to find examples of acoustic solutions and products available on the market?

Visit the Acoustic Design Catalogue by clicking on the button below.

access the catalogue here

What is a sound absorber ?

A succinct definition of a sound absorptive material is:

“A sound absorptive material is a material that acts as a ‘noise sponge‘ by converting the sound energy into heat within the material.”

(from Environmental Impact Assessment, Theory and Practice – Anji Reddy Mareddy)

When sound strikes a material, some of its energy converts to heat, reducing the reflected sound energy.

The sound absorption of construction materials typically spans across narrow or wide frequency ranges, not single frequencies.

(if you need to understand what frequencies are read this article, and frequency bands read this article)

Various factors influence a material’s sound absorption in a space:

the material type (hard, fibre-based, porous, etc)
the thickness
the finish (painted, cloth-covered, plastered, etc)
the placement in the space
the mounting method (on/off hard surface, free-hanging, etc)

Common sound absorptive solutions in construction use porous, fibrous, or cellular materials.

Other solutions utilize these materials with additional features like cavities, perforations or microperforations to enhance the sound absorption across specific frequency ranges.

Examples of materials will be presented in future posts.

How do we rate the sound absorption capacity of materials ?

For most construction materials, a common index to rate the sound absorption is the sound absorption coefficient , termed α (‘alpha’).

Note

It is officially called the Practical Sound Absorption Coefficient, termed α_p, in the standard ISO 11654 (2023) – Sound absorbers for use in buildings – Rating of sound absorption.

This coefficient is the ratio of:

the sound energy absorbed (Ea) by a material;
divided by the sound energy incident (Ei) on the same material.

As the absorbed sound energy (Ea) increases, the sound absorption coefficient increases too.

Conversely, a decrease in absorbed sound energy results in a lower absorption coefficient.

Thus, better sound absorption yields a higher absorption coefficient, logically.

Note

It’s worth also noting that Ea can’t physically be higher than Ei (there can’t be more absorbed energy than there is incident energy).

Hence Ea ≤ Ei always, ensuring that the sound absorption coefficient is always between 0 and 1.

However, coefficients higher than 1 may occur due to testing methods or calculations.

Other methods and indices for evaluating sound absorption of building materials include:

the Noise Reduction Coefficient (NRC) and Sound Absorption Average (SAA) as per ASTM C423 – Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method
the Weighted Sound Absorption Coefficient (α_w) as per ISO 11654 (2023).
the Sound absorption class (ranging from E to A, with A being the highest), as per ISO 11654 (2023)
the Equivalent sound absorption area.

The latter two will be elaborated in future posts.

How do we measure the sound absorption coefficient of materials ?

There are two main methods to measure the sound absorption coefficient of a material:

the Impedance Tube Method.
the Reverberation Chamber Method or Room Method.

The Impedance Tube Method

This method involves placing a sample of material at the end of a small tube.

Sound is emitted into the tube at different frequencies.

The sound absorption coefficient of the material is then derived by comparing the sound in the tube before and after it strikes the sample.

While more practical and time-efficient, this method is not preferred due to its restrictions on the type of material and mounting conditions.

Note

Under the impedance tube method, there are actually two sub-methods:

the standing wave ratio method and;
the transfer function method.

Both are prescribed by normative regulations outlined in the following standards:

ISO 10534-1 (2022) – Determination of sound absorption coefficient and impedance in impedance tubes – Part 1: Method using standing wave ratio.
ISO 10534-2 (2023) – Determination of sound absorption coefficient and impedance in impedance tubes – Part 2: Transfer-function method.
ASTM E 1050-19 – Standard test method for impedance and absorption of acoustical materials using a tube, two microphones and a digital frequency analysis system.

The Reverberation Chamber or Room Method

This method involves placing large samples or multiple samples of the material to be tested in a large & highly reverberant room.

The room typically comprises hard surfaces (walls, ceiling, and floor) that reflect sound, absorbing minimal to no sound, along with large sound reflective surfaces hung from the ceiling and sometimes positioned near the walls.

This setup aims to scatter sound extensively, creating a diffusive sound field within the room. This enables simulation of sound reaching the samples from all directions.

The sound absorption coefficient is calculated by comparing:

Sound reverberation measurements without any materials.
Sound reverberation measurements with the material under test.

Note

This measurement methodology is prescribed in EN ISO 354 (2003) – Acoustics Measurement of sound absorption in a reverberation room mention.

While not as practical or time-effective as the impedance tube method, this approach offers the advantage of testing materials in more realistic mounting conditions. These include (but not only):

Mounted directly against a hard surface.

Hung freely.

Positioned within a cavity.

Split into small samples.

And many others

Note

EN ISO 354 also provides guidance (in Appendix B) on the

appropriate mounting of the test sample whether it is:

mounted directly against a surface of the reverberant room (Types A and B).
mounted with an airspace behind it (Type E)
hung parallel to a surface of the reverberant room (Type G)
spray- or trowel-applied to a substrate , usually sound absorptive (Type I)
with one edge resting on or touching a room surface (Type J)

.

We will provide examples of sound absorption coefficients for different construction materials in future posts.

References

See below the references used to confirm the above concepts:

Noise Control in Building Services – Sound Research Laboratories Ltd – Pergamon Press
Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers
Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition – Trevor Cox, Peter D’Antonio
Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen
Mechanical and Physical Testing of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites – (A volume in Woodhead Publishing Series in Composites Science and Engineering) Mohammad Jawaid, Mohamed Thariq & Naheed Saba – Woodhead Publishing – 2019

Sound propagation, reflection, absorption and transmission

Posted on 5 March 20245 March 2024 by ateliercrescendo

Sound propagation, reflection, absorption and transmission

Quite often, we receive questions on how sound propagates, especially through materials.

If you need to understand the difference between sound reflection, sound absorption and sound transmission, here is a small post to explain the principles.

Enjoy the read!

(See at the end the references used to confirm these concepts)

PS: if you need to understand some basic concepts of sound like wavelengths and frequencies, read fundamental concepts of sound.

How does sound propagate?

Assuming a single sound source, typically called a point source, emitting sound uniformly (i.e. the same way in all directions).

In the near field, it produces sound waves that spread outward from the source in a spherical pattern.

Note: the near field is a region close to the source

In the far field, sound continues to propagate outward from the source. However, it appears to propagate more in a line pattern.

Note: the far field is a region far from the source

To aid visualization, we often use rays to represent how sound propagates. They clearly indicate the direction of a specific sound’s propagation.

Throughout the remainder of this post, as well as in many others, we illustrate sound propagation using rays.

What are sound reflection, absorption and transmission?

(and also sound insulation and soundproofing)

Assuming a material that is infinitely long and flat, with no thickness considered.

When sound propagates at a certain angle from the surface, the angle between its direction and the surface’s normal is called the angle of incidence.

As sound interacts with this material, three phenomena occur.

Sound is reflected

at the same angle as the angle of incidence.

We call it the angle of reflection.

Sound is absorbed

losing acoustic energy transformed into heat within the material.

Sound is transmitted

to the other side of the material.

A material’s ability to reduce the sound transmission is its sound insulation capacity

(or also soundproofing capacity).

This capacity varies depending on the material’s physical properties, affecting how sound is reflected, absorbed, and transmitted.

Additionally, sound reflection, absorption, and transmission vary across different frequencies.

(if you need to understand the concept of frequencies, read the fundamental concepts of sound)

References

See below the references used to confirm the above concepts:

Noise Control in Building Services – Sound Research Laboratories Ltd – Pergamon Press
Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers
Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition – Trevor Cox, Peter D’Antonio
Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen

Understanding frequency bands

Posted on 5 February 202429 January 2024 by ateliercrescendo

Understanding frequency bands

In fundamental concepts of sound, we covered some basic concepts like wavelengths and frequencies.

Acoustic experts often use frequency bands in their studies.

For non-experts, understanding concepts like third-octave and octave frequency bands can be tricky.

If you find this concept challenging, worry not! Atelier Crescendo has crafted this post just for you.

Enjoy the read!

(See at the end the references used to confirm these concepts)

Acoustic analysis

Studying the acoustic performance of materials or the acoustic characteristics of sources/signals comes in different ways.

Frequently, acoustic experts use frequency bands for measurements or calculations.

Essentially, it is a method to reveal acoustic information across different frequency ranges.

Most commonly, frequency analysis occurs in:

octave bands
third-octave bands

In third-octave bands, the precision/resolution is about three times higher than in octave bands.

The formula to obtain octave band values from third octave values depends on the acoustic metric you want to study.

What is an octave ?

An octave is an (key musical) interval, corresponding to doubling or halving frequency.

Example: from 125 Hz to 250 Hz is an octave, as is 20,000 Hz to 10,000 Hz.

Musicians often use the A note as the reference for tuning. The frequency of this note is 440 Hz. The A note at the next octave is 880 Hz.

Upper & higher frequency band limits

and band centre frequency

When talking about bands, we must set their upper and lower frequency limits.

Also, in each band, there is a center, known as the band center frequency. It is the mean of the upper and lower limits.

Example: in the 250 Hz center frequency octave band:

the lower limit is 178 Hz
the upper limit is 355 Hz

Setting limits for third-octave bands is a little more complex.

Preferred (and standardised) frequency bands

Frequency band analysis has even been standardised.

International Standards Organisations have agreed on preferred frequency bands for the analysis of sound or vibrations.

Typically, in acoustic building projects, the frequency range under consideration spans from 63 Hz to 8000 Hz.

In the case of vibrations, exploration can extend to frequencies as low as 2 Hz.

Note: If you want to know about the standards, here they are:

ANSI/ASA S1.6 (2020) – Preferred Frequencies And Filter Band Center Frequencies For Acoustical Measurements
ANSI/ASA 51.11 (2004) – Octave-Band And Fractional-Octave-Band Analog And Digital Filters
IEC 61260 -1 (2014) – Electroacoustics – Octave-band and fractional-octave-band filters – Part 1: Specifications
TS0 266 (1997) – Acoustics – Preferred frequencies

References

See below the references used to confirm the above concepts:

Noise Control in Building Services – Sound Research Laboratories Ltd – Pergamon Press
Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers
Acoustic Absorbers and Diffusers – Theory, Design and Application – Third Edition – Trevor Cox, Peter D’Antonio
Engineering Noise Control – Sixth Edition – CRC Press – David A. Bies, Colin H. Hansen, Carl Q. Howard, Kristy L. Hansen

Deep into wavelengths, frequencies and spectrums of instruments

Posted on 25 January 202425 January 2024 by ateliercrescendo

Deep into wavelengths, frequencies and spectrums of instruments

This post is not very long.

It is a complement to the fundamental concepts of sound in which we explained about sound, sound waves, amplitude, wavelengths and frequencies.

Here, we firstly give you some ideas on the dimensions of wavelengths for different frequencies in the air. They are useful to know to better understand sound phenoma like: sound absorption, sound diffusion, sound diffraction and also sound transmission.

Then we have included a graph with examples of spectrum of some instruments and sources. This way, you can visualise the frequencies they can emit.

Wavelength dimensions

Below we illustrate wavelength dimensions for various frequencies within the audible range for humans.

These examples offer insights into the sizes of wavelengths associated with the sounds/frequencies we can hear.

Note: We have approximated the wavelength values by calculating them under the assumption of air as the propagation medium, at a temperature of 20 degrees Celsius, resulting in a speed of sound at 343 meters per second.”

Examples of wavelength dimensions corresponding to different frequencies

Frequency

Corresponding Wavelength

20 Hz

17,15 meters

50 Hz

6,86 meters

100 Hz

3,43 meters

500 Hz

69 cm

1000 Hz

34 cm

5000 Hz

6,9 cm

10000 Hz

3,4 cm

20000 Hz

1,7 cm

Examples of spectrums

Remember from the fundamental concepts of sound, “the natural sounds blend frequencies with varying amplitudes.”

We call spectrum (in physical terms) the range of frequencies a source can emit.

Below we have included a graph with examples of spectrums of some instruments and sources.

Fundamental concepts of sound (sound waves, amplitude, wavelength, frequency)

Posted on 9 January 202425 January 2024 by ateliercrescendo

Fundamental concepts of sound

(Sound waves, amplitude, wavelength, frequency, etc)

Are you new to acoustics or need a refresher on sound basics?

This article was created to dive into the fundamental concepts of sound including:

Sound and sound waves
Sound wavelength
Sound amplitude
Sound frequency

Enjoy the read!

(See at the end the references used to confirm these concepts)

Understanding sound and sound waves

Sound originates from vibrating objects in various mediums like solids, liquids, and gases.

These vibrations cause air particles to move back and forth, creating compression and rarefaction.

Compression occurs when particles bunch together, generating a higher pressure.

Conversely, rarefaction leads to lower pressure as particles spread apart.

These alternating pressure fluctuations give rise to the sounds we perceive.

These pressure variations form sound waves, classified as longitudinal waves because the air particles move in the same direction as the vibrations.

In air, sound waves travel at around 343 meters per second (at 20°C).

Understanding wavelength in sound waves

Wavelength in sound waves refers to the distance between two consecutive compressions or rarefactions of particules.

Typically, wavelengths are expressed in meters.

Understanding amplitude

Sound amplitude reflects the change in pressure from vibrations. Put simply, it relates the number of air particles involved in the vibration process.

A sound with a greater amplitude will be perceived as louder (or with a higher volume).

To measure amplitude, you use logarithmic decibels (dB).

For instance, a whisper might register around 30 dB, while a rock concert can reach a staggering 120 dB or higher.

Understanding frequencies

Frequencies, measured in Hertz (Hz), directly relate to sound wave speed divided by the wavelength.

shorter wavelengths correspond to higher frequencies…

while larger wavelengths align with lower frequencies.

Higher frequencies result in higher pitches, while lower frequencies create lower pitches. Listen below to some examples perfect pitches at different frequencies.

100 Hz pitch

500 Hz pitch

1000 Hz pitch

10000 Hz pitch

The audible range for most humans falls between 20 Hz to 20,000 Hz.

Natural sounds blend frequencies with varying amplitudes, forming a spectrum.

The unique timbre of sources, especially musical instruments, links closely to their individual spectra.

References

See below the references used to confirm the above concepts:

Noise Control in Building Services – Sound Research Laboratories (SRL) Ltd – Pergamon Press
Auditorium Acoustics and Architectural Design (Second Edition) – Michael Barron – Spon Press
Sound Materials, A Compendium of Sound Absorbing Materials for Architecture and Design – Tyler Adams – Frame Publishers

Noise and vibration challenges in gyms and sports facilities – Part 2: Control of airborne noise

Posted on 17 July 202318 July 2023 by ateliercrescendo

Noise and vibration challenges in sport, fitness and gym facilities

Part 2: Control of airborne noise

In sport, fitness and gym facilities, many activities and sources generate airborne noise including:

Power amplified sound systems
Exercise machines
Weights dropped (controlled or uncontrolled) on structures
People chatting, cheering, shouting, etc.
Other noises specific to some sports (i.e. hockey, tennis, football, etc.)

This part explains the different paths of airborne noise transmission from the gym spaces to the adjacent/nearby noise sensitive receptors.

For each transmission path, it explains what happens and gives ideas of noise control solutions.

Note 1: what is airborne noise exactly? go to this section in part 1.

Note 2 : read Who could the facility disturb iin part 1 to know what the noise sensitive receptors could be.

Airborne noise transmission paths in sport, fitness and gym facilities

The main transmission paths of airborne noise are:

through the building façace.
through the internal building fabric and;
via the ventilation systems.

It is also useful to control the sound reverberation in noisy spaces.

Follow the links to read what happens and know ideas of noise control solutions for each transmission path.

Noise transfer through the building façade
Noise transfer through the internal building fabric
Noise transfer via the ventilation systems
Control the sound reverberation

Noise transfer through the building façade

of sport, fitness and gym facilities

What happens?

In sport, fitness and gym facilities, activity and music noise:

transfers through the façade,
and propagates through the air to the nearest sensitive receptor.

Local Authorities often ask to consider this aspect to make sure the new facility doesn’t cause an adverse impact on the neighbours.

To do this, you need to undertake a noise impact assessment.

Note: Most of the time, music and activity noise impact assessments aim to not exceed (to a degree) the existing noise environmental noise levels at the receptors. There are called ‘subjective assessments’.

But sometimes, undertaking a subjective assessment is not the most ‘reasonable’ approach.

For very quiet sites, exceeding slightly the existing noise environmental noise levels may not cause an adverse impact.

In this case, undertaking an assessment based on not exceeding (again, to a degree) an objective requirement may be more appropriate. This requirement, usually from a guideline or a standard, has to be discussed and agreed with the Local Authority.

You call such as assessment an ‘objective assessment’.

What are the noise control solutions?

There are two main solutions to control music and activity noises in sport, fitness and gym facilities.

Solution 1:

Improve the façade elements and/or the roof

You generally do it by creating cavities with dense & rigid board elements such as:

plasterboard lining on a frame with insulation in the cavity to improve the performance of the external walls.

You may need to decouple it from the rest of the building structure by introducing resilient fixings or, when possible, simply make the frame independant.

Examples of resilient fixings are resilient bars or resilient clips.

plasterboard ceiling on a frame with insulation in the cavity to improve the performance of the roof.

You may need to decouple it from the rest of the building structure by introducing resilient fixings.

As above, examples of resilient fixings are resilient bars, clips or hangers.

if the façade contains glazed elements, you may need to think about high performance double or triple glazing.

Sometimes, secondary glazing is necessary when you need to increase the sound insulation performance at low frequencies.

Solution 2:

Install a noise limiter

A noise limiter is usually a secondary solution.

It is a device that limits (!!) the music levels emitted within a space to avoid too much noise spilling out of a facility.

This way the music levels stay below certain thresholds at the nearest noise sensitive receptors.

Note: the word thresholds here is with an “s” because you set the noise limiter in different frequency bands (or ranges).

Noise transfer through the internal building fabric

of sport, fitness and gym facilities

What happens?

In a building, the airborne noise transfers through the wall/floor constructions into adjacent spaces.

What are the noise control solutions?

Depending of the site and the context, you may have to implement some or all the following noise control solutions.

Solution 1:

Improve the floors

You can improve the airborne sound insulation performance of a floor construction with:

a dense suspended ceiling that you decouple from the main structure with resilient fixings such as resilient bars, clips or hangers.

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Note: you may also need to consider a floating floor system in the gym.

However, this is more to control the impacts of weights dropped and vibrations of some activities & equipment (see future parts on vibration isolation).

Solution 2:

Improve the walls

You can improve the airborne sound insulation performance of external wall constructions in vary many different ways.

However, these two systems are recurrent when receptors are fairly close (< 20m) to the facilities :

Drylining or ‘sandwich‘ systems. Most of the time, they include plasterboard on the indoor side.

Dense linings to masonry walls.

For both methods, it might be useful to think about introducing:

a larger cavity to better control low frequency transmission.
some decoupling elements to increase their performance such as
- acoustic studs.
- resilient clips.
- resilient bars.
- or even make the studs independant from the outer structure.

Note: If the facility is to move in an existing building, sound insulation testing are necessary to rate the performance of the separations elements and work out the best solution for improvement.

Note: sometimes, a full box-in-box construction will be necessary

Noise transfer via the ventilation systems

of sport, fitness and gym facilities

What happens?

The music and activity noises from sport, fitness and gym facilities:

‘enter’ the ventilation systems
propagate through the ducts, and;
break outside or in another space within the same building (although this last one is unlikely because most facilities have their dedicated ventilation system).

What are the noise control solutions?

To control the noise transfer via a ventilation system, you can think about the following solutions.

Solution 1:

Install attenuators

This is probably the most common and easiest solution.

Their performance mainly depends on their length and their free area.

However, be aware that attenuators shouldn’t be installed anywhere in the ductwork for optimal acoustic performance.

The best locations to install the attenuators are:

centered in the wall separating the noisy space and the rest of the building;

or at the wall separating the noisy space and the rest of the building.

Solution 2:

Strategically locate the inlets and outlets

Ideally, you should locate the inlets and oulets of a ventilation system as far away from the sensitive receptors as possible. (although some sites/buildings don’t offer a great amount of flexibility).

This way, you can reduce the performance of the attenuators (and save some cost)… or completely omit them.

Solution 3:

Select larger and square ducts (for low frequency attenuation)

If you need to attenuate low frequency sounds, you could think about selecting large and square ducts.

Note: This is only worth if:

you have long runs of ductwork.
you have enough space for large ducts.

Solution 4

Select lined ducts

Lined ducts attenuate more noise than unlined ducts.

Therefore, you can use them to attenuate some amount of music and activity noise.

Note: it is only worth if:

you have long runs of ductwork.
the pressure drop they create doesn’t require a higher air flow (which would be counter productive). This needs to be checked with the mechanical engineer on board.

Control of sound reverberated (i.e. reflected)

in sport, fitness and gym spaces

What happens?

Within most sport, fitness and gym spaces, medium to high levels of sound are generated.

With a majority of hard surfaces (i.e. sound reflecting), the spaces can be very reverberant and amplify these sounds.

This also makes it harder to control the noise transfer from the gym to other receptors (within or outside of the same building).

What is the noise control solution?

To reduce that reverberation effect, you need to include sound absorptive finishes on the walls and the ceiling.

Where possible, it is useful to install a carpet to contribute to the sound absorption.

Noise and Vibration challenges in sport, fitness and gym facilities – Part 1: Site analysis

Posted on 14 November 20227 February 2023 by ateliercrescendo

Noise and vibration challenges in sport, fitness and gym facilities

Part 1: Site Analysis

In sport, fitness and gym facilities, you find a lot of activities and equipment that create noise and vibrations.

If not controlled correctly, they can disburb neighbouring sites, neighbouring premises, and even the facility itself.

Atelier Crescendo have created this series of posts to make you understand:

which activities are disturbing.
how they are disturbing.
who / what they can be disturing to.
what the solutions are to reduce the noise and vibration disturbance.

This post suggests the first step of such a process: a complete and thorough Site Analysis.

Site analysis of sport, fitness and gym facilities

A complete and thorough site analysis of a facility serves the design of efficient and cost effective acoustic solutions.

Therefore, this first post lists a few questions to ask yourself for a site analysis and explains why they are important .

(click on each link to access the relevant section)

What are the activities proposed?
Who could the facility disturb? (i.e. what are the noise sensitive receptors?)
Where are the noise sensitive receptors?
When will the facility operate?
What is the structure of the facility?
What is the layout of the facility? (i.e. where will the different activities take place)

What are the activities proposed?

Various activities and equipment in sport, fitness and gym facilities produce noise and/or vibrations.

They do it in very different ways and at different intensities, leading to require very specific control solutions.

This section gives you a few examples of such activities and equipment.

Loud amplified sound

Most fitness and gym facilities play background music which is generally not a problem.

However, loud music (or sounds) may be played in some areas such as:

activity and dance studios
sports halls during events
external sport areas during events
others

Therefore, knowing which areas will produce loud amplified sound is important to take into account in the site analysis.

Vibrating machines

Some equipment that including rotating system (of some sort) not only produce noise but also vibrations. This includes:

treadmills
bikes
ellipticals
others

The above equipment produces (relatively) mild but ‘constant’ vibrations that are transmitted to the building structure.

Resistance machines

Most resistance machines involve lifting a stack of weights indirectly by performing a specific movement. They include:

chest press
leg press
lat pulldown
cable biceps/triceps bar
seated row
shoulder press
others

Each time the stack drops, it creates an impact. This impact produces vibrations that are then transmitted to the building structure.

Functional training and sport areas

Sport, fitness and gym facilities sometimes include functional training and/or sport areas.

All these activities involve medium (up to 20-30 kg) or body weights dropped from heights that usually don’t exceed knee height (i.e. about 0.5 m).

They create repeated and (sometimes) synchronised impacts that produce vibrations of ‘medium intensity’.

They include:

running tracks
slam ball/wall ball work
sports pitches
sports courts (tennis)
sled training
TRX training
kettlebell training
others

Free weight areas

Free weight areas are probably the most challenging areas to isolate in sport, fitness and gym facilities.

Most of the time, they involve:

using medium (up to 20-30 kg) to very heavy (more than 50 kg) weights.
lifting them from knee to above head height.
Dropping them on the floor in, most of the time, an uncontrolled manner.

This process creates a very strong impact, with a lot of energy transmitted, that can be very difficult to mitigate.

Who could the facility disturb?

(i.e. what are the noise sensitive receptors)

What is a noise sensitive receptor?

It is a property or a space located within the same building as the gym/sport facility or close to the site.

Examples of such receptors could be:

Residential properties
Commercial properties (i.e. offices, retail, etc)
Education facilities
Facilities with equipment sensitive to vibrations (such as laboratories, precision engineering facilities, etc)

noise disturbance - vibration disturbance - noise sensitive receptors - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

One of the reasons why a site analysis is important is to help set relevant target for the acoustic treatment(s) should achieve.

This target, or also criterion, highly depends on the who/what the facility is likely to disturb.

Some noise sensitive receptors are more sensitive than others (see beside what a noise sensitive receptor is). For example:

dwellings are generally more sensitive to gym noise than commercial properties (especially at night).
Offices are more sensitive than retail premises.
Some laboratories may have equipment sensitive to levels of vibrations that are not perceptible by the human body.

For residential receptors, you usually discuss and agree with the Local Authority. For other receptors, you could agree a criteria with the landlord or the neighbouring facilities.

Therefore, understanding who your sport, fitness and gym facility might disturb will help define the level of noise and/vibration mitigations.

Where are the noise sensitive receptors?

Will they be disturbed by airborne noise, structure-borne noise or both?

Knowing where the noise sensitive receptors are allows you to know if the cause of the disturbance is:

Just airborne noise

Note: What is airborne noise? it is noise that only propagates through the air or the atmosphere.

It is the case when the facilities and the receptors are structurally disconnected (i.e. not part of the same building).

airborne noise - amplified music - loud music - loud activities - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration — **Airborne noise** of sport, fitness and gym facilities

Just ‘structure-borne’ noise

Note: What is ‘structure-borne’ noise? it is noise generated by vibrations that propagate through a structure (mostly building structures here) and are re-radiated into noise.

It is the case when the facilities and the receptors are structurally connected (i.e. they are part of the same building) but are fairly remote from one another (i.e. not adjacent)

Structure-borne noise - structure borne noise - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration — **Structure-borne noise** of sport, fitness and gym facilities

Or both airborne noise and ‘structure-borne’ noise (and maybe pure vibration as well):

It is the case when the facility and the receptors are structurally connected and are close to one another (or even adjacent!).

airborne noise - Structure-borne noise - structure borne noise - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration — **Structure-borne and airborne noise** of sport, fitness and gym facilities

The relevant acoustic treatment to control the noise and/or vibrations from the facility varies for each situation above.

When will the facility operate?

Knowing when (i.e. which periods of the day) a sport, fitness and gym facility will operate

is very important to set relevant noise and/vibration control criteria.

Choosing to operate a facility, with loud music and/or heavy impacts on the floor, during late evening and night time periods is likely to increase the cost of acoustic treatment needed.

Especially, when the facility is near or adjacent to dwellings.

Note: Late evening is approximately after 19:00 hours. Night time is approximately between 23:00 and 07:00 hours.

The reason of this is because you need to set very strigent noise/vibration criteria for the activities to not cause any disturbance to neighbours needing to rest or sleep.

External noise levels are also lower between 19:00 and 07:00 hours, so the activities or the music are more audible to neighbours.

Note: This section might be redundant as a lot of sport, fitness and gym facilities operate 24/7 and their peak hours are generally early in the morning (between 06:00 and 08:00 hours) and in the evening (between 17:00 and 20:00 hours)

operating hours - gym operation - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

What is the structure of the building where the facility is?

The structure of a building has a big influence in the acoustic treatment required.

Structure impact - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

The structure of the building influences:

The sound insulation treatment for the floors and the walls.
The vibration isolation treatment for the floors.

And you will generally need less material for heavy and stiff structures than for light structure.

Examples of heavy structures are in-situ concrete structures, brickwork, etc.

Examples of light structures are timber, mass timber, pre-cast concrete and metal frame structures.

Note: If you want to install a free weight area on a timber structure, it will be very challenging, or expensive, to control the ‘structure-borne‘ noise generated by the impacts of the weights on the floor.

(see above Section Where are the noise sensitive receptors? above for the definition of ‘structure-borne‘ noise)

What is the layout of the facility?

(i.e. where will the different activities go)

Planning the facilities layout is an integral part of noise and vibration control strategy.

It may be obvious, but:

the further you locate the noisy and vibrating activities from the sensitive receptors,

the less materials and acoustic treatment you need.

So by just optimising the layout of a facility, you can already control the noise and vibration impacts of the activities.

Below are three types of areas that are worth considering for such as process.

Note: The last two points are particularly useful to know if you need to select different sites and assess their suitability for sport, fitness and gym use.

football pitch - hockey pitch - external sport areas - sport pitches- running track - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

External running tracks, sport pitches and other areas

Some external sport areas produce noise that can be particularly disturbing when they are close to sensitive receptors.

Therefore, by locating these areas strategically, you can avoid some solutions like acoustic fences or earth bunds.

There are two main strategies here. Either you locate the areas:

away from the receptors, or;
‘behind’ the some buildings (that of the facilities for example) that you use as acoustic screens.

Obviously, they are very much site dependant and sometimes can’t be implemented.

studios - dance studio - sports hall indoor areas - sports halls - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

Studios, sports halls and other indoor areas

Most studios, sports hall and other indoor areas organise noisy activities or events with loud amplified sounds.

They can be particularly disturbing to receptors within the same building or in a neighbouring building.

Although you can control noise (and vibrations!) from these areas with the finishes and the construction of the building. It is also worth thinking about locating them as far away as possible from the receptors … when possible.

Another solution is to use secondary spaces (like bathrooms, stores, locker rooms, etc) as ‘buffer’ areas between the noisy spaces and the receptors.

free weight areas - barbell - kettlebell - treadmill - elliptical - gym bike - Vibrating machines - Noise and vibration challenges in sport, fitness and gym facilities Part 1: Site Analysis - acoustic consultant - noise control - gym acoustics - vibration control - gym vibration

‘Vibrant’ equipment and activities

The vibrations generated by some equipment and activities (such as free weight activities, skate parks and even sports courts and studios) can be particuarly complicated and/or expensive to control.

Therefore, it will always be useful to think where they can go in the facilities to minimise the amounts of anti-vibration materials.

at ground floor level and/or on heavy slabs.
As far away as possible from the sensitive receptors.

Solid free hanging panels can absorb sound

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Visit the Acoustic Design Catalogue by clicking on the button below.

Why can solid free hanging panels absorb sound ?

Note:

You may need to read Fundamental concepts of sounds to understand the relationship between frequencies and the motion of air particules.

How to specify solid free hanging panels?

Note:

Why is it useful to calculate the resonant frequency of a panel?

Because it is the frequency at which a panel moves the most and absorbs the most sound.

Influence of the

mass ‘m’

Influence of the

stiffness ‘B’

Influence of the panel

dimensions

‘a’ and ‘b’

Porous sound absorbers

explained

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Visit the Acoustic Design Catalogue by clicking on the button below.

What are porous sound absorbers ?

Note

The shape and dimensions of the pores and fibres also significantly influence the sound absorption

Porosity and air flow resistivity of porous sound absorbers ?

Porosity of some construction materials

Sound absorption of fibreglass (53 kg/m3 density) samples of 25 mm, 50 mm, 75 mm and 100 mm thicknesses (fixed to a hard surface)

Why is that

With increasing thickness, more material exists away from the surface, where the particules’ velocity (i.e. energy) is higher than at the wall.

This leads to enhanced absorption, particularly for lower frequencies (or larger wavelengths).

Sound absorption of mineral wool of 16 kg/m3 , 24 kg/m3, 48 kg/m3 and 60 kg/m3 densities (50 mm fixed to a hard surface)

Influence of the mounting conditions on the sound absorption?

Sound absorption of acoustic tiles directly applied to a hard surface or suspended

Influence of the cover on the sound absorption?

Influence of paint treatment on the absorption of a sound absorber

Sound absorption classes

explained

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What are sound absorption classes ?

Note

You can find the details of this classification in ISO 11654 (2023) – Sound absorbers for use in buildings – Rating of sound absorption.

How are sound absorption classes established ?

From frequency-dependent performance to single number value

Note 1:

An unfavorable deviation occurs when, for a given octave, the measured value is

less

than the value of the reference curve.

And vice versa, a ‘favourable’ deviation occurs when, for a given octave, the measured value is

more

than the value of the reference curve.

Note 2:

In the calculations, we only consider

unfavourable deviations.

From single number performance to sound absorption class

Sound absorption basics

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What is a sound absorber ?

“A sound absorptive material is a material that acts as a ‘noise sponge‘ by converting the sound energy into heat within the material.”

(from Environmental Impact Assessment, Theory and Practice – Anji Reddy Mareddy)

How do we rate the sound absorption capacity of materials ?

Note

It is officially called the Practical Sound Absorption Coefficient, termed αp, in the standard ISO 11654 (2023) – Sound absorbers for use in buildings – Rating of sound absorption.

Note

It’s worth also noting that Ea can’t physically be higher than Ei (there can’t be more absorbed energy than there is incident energy).

Hence Ea ≤ Ei always, ensuring that the sound absorption coefficient is always between 0 and 1.

However, coefficients higher than 1 may occur due to testing methods or calculations.

How do we measure the sound absorption coefficient of materials ?

The Impedance Tube Method

Note

Under the impedance tube method, there are actually two sub-methods:

the standing wave ratio method and;

the transfer function method.

Both are prescribed by normative regulations outlined in the following standards:

ISO 10534-1 (2022) – Determination of sound absorption coefficient and impedance in impedance tubes – Part 1: Method using standing wave ratio.

ISO 10534-2 (2023) – Determination of sound absorption coefficient and impedance in impedance tubes – Part 2: Transfer-function method.

ASTM E 1050-19 – Standard test method for impedance and absorption of acoustical materials using a tube, two microphones and a digital frequency analysis system.

The Reverberation Chamber or Room Method

Note

Sound absorption of fibreglass (53 kg/m³ density) samples of 25 mm, 50 mm, 75 mm and 100 mm thicknesses (fixed to a hard surface)

Sound absorption of mineral wool of 16 kg/m³ , 24 kg/m³, 48 kg/m³and 60 kg/m³densities (50 mm fixed to a hard surface)

It is officially called the Practical Sound Absorption Coefficient, termed α_p, in the standard ISO 11654 (2023) – Sound absorbers for use in buildings – Rating of sound absorption.